bpf: Allow kind_flag for BTF type and decl tags

BTF type tags and decl tags now may have info->kflag set to 1,
changing the semantics of the tag.

Change BTF verification to permit BTF that makes use of this feature:
  * remove kflag check in btf_decl_tag_check_meta(), as both values
    are valid
  * allow kflag to be set for BTF_KIND_TYPE_TAG type in
    btf_ref_type_check_meta()

Make sure kind_flag is NOT set when checking for specific BTF tags,
such as "kptr", "user" etc.

Modify a selftest checking for kflag in decl_tag accordingly.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20250130201239.1429648-6-ihor.solodrai@linux.dev

2 files changed, 13 insertions, 17 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 9de6acddd479..9433b6467bbe 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2575,7 +2575,7 @@ static int btf_ref_type_check_meta(struct btf_verifier_env *env,
 		return -EINVAL;
 	}
 
-	if (btf_type_kflag(t)) {
+	if (btf_type_kflag(t) && !btf_type_is_type_tag(t)) {
 		btf_verifier_log_type(env, t, "Invalid btf_info kind_flag");
 		return -EINVAL;
 	}
@@ -3332,6 +3332,8 @@ static int btf_find_kptr(const struct btf *btf, const struct btf_type *t,
 			 u32 off, int sz, struct btf_field_info *info, u32 field_mask)
 {
 	enum btf_field_type type;
+	const char *tag_value;
+	bool is_type_tag;
 	u32 res_id;
 
 	/* Permit modifiers on the pointer itself */
@@ -3341,19 +3343,20 @@ static int btf_find_kptr(const struct btf *btf, const struct btf_type *t,
 	if (!btf_type_is_ptr(t))
 		return BTF_FIELD_IGNORE;
 	t = btf_type_by_id(btf, t->type);
-
-	if (!btf_type_is_type_tag(t))
+	is_type_tag = btf_type_is_type_tag(t) && !btf_type_kflag(t);
+	if (!is_type_tag)
 		return BTF_FIELD_IGNORE;
 	/* Reject extra tags */
 	if (btf_type_is_type_tag(btf_type_by_id(btf, t->type)))
 		return -EINVAL;
-	if (!strcmp("kptr_untrusted", __btf_name_by_offset(btf, t->name_off)))
+	tag_value = __btf_name_by_offset(btf, t->name_off);
+	if (!strcmp("kptr_untrusted", tag_value))
 		type = BPF_KPTR_UNREF;
-	else if (!strcmp("kptr", __btf_name_by_offset(btf, t->name_off)))
+	else if (!strcmp("kptr", tag_value))
 		type = BPF_KPTR_REF;
-	else if (!strcmp("percpu_kptr", __btf_name_by_offset(btf, t->name_off)))
+	else if (!strcmp("percpu_kptr", tag_value))
 		type = BPF_KPTR_PERCPU;
-	else if (!strcmp("uptr", __btf_name_by_offset(btf, t->name_off)))
+	else if (!strcmp("uptr", tag_value))
 		type = BPF_UPTR;
 	else
 		return -EINVAL;
@@ -4944,11 +4947,6 @@ static s32 btf_decl_tag_check_meta(struct btf_verifier_env *env,
 		return -EINVAL;
 	}
 
-	if (btf_type_kflag(t)) {
-		btf_verifier_log_type(env, t, "Invalid btf_info kind_flag");
-		return -EINVAL;
-	}
-
 	component_idx = btf_type_decl_tag(t)->component_idx;
 	if (component_idx < -1) {
 		btf_verifier_log_type(env, t, "Invalid component_idx");
@@ -6743,7 +6741,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	info->btf_id = t->type;
 	t = btf_type_by_id(btf, t->type);
 
-	if (btf_type_is_type_tag(t)) {
+	if (btf_type_is_type_tag(t) && !btf_type_kflag(t)) {
 		tag_value = __btf_name_by_offset(btf, t->name_off);
 		if (strcmp(tag_value, "user") == 0)
 			info->reg_type |= MEM_USER;
@@ -7002,7 +7000,7 @@ error:
 
 			/* check type tag */
 			t = btf_type_by_id(btf, mtype->type);
-			if (btf_type_is_type_tag(t)) {
+			if (btf_type_is_type_tag(t) && !btf_type_kflag(t)) {
 				tag_value = __btf_name_by_offset(btf, t->name_off);
 				/* check __user tag */
 				if (strcmp(tag_value, "user") == 0)
diff --git a/tools/testing/selftests/bpf/prog_tests/btf.c b/tools/testing/selftests/bpf/prog_tests/btf.c
index e63d74ce046f..aab9ad88c845 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf.c
@@ -3866,7 +3866,7 @@ static struct btf_raw_test raw_tests[] = {
 	.err_str = "vlen != 0",
 },
 {
-	.descr = "decl_tag test #8, invalid kflag",
+	.descr = "decl_tag test #8, tag with kflag",
 	.raw_types = {
 		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, 4),	/* [1] */
 		BTF_VAR_ENC(NAME_TBD, 1, 0),			/* [2] */
@@ -3881,8 +3881,6 @@ static struct btf_raw_test raw_tests[] = {
 	.key_type_id = 1,
 	.value_type_id = 1,
 	.max_entries = 1,
-	.btf_load_err = true,
-	.err_str = "Invalid btf_info kind_flag",
 },
 {
 	.descr = "decl_tag test #9, var, invalid component_idx",